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4. Global Atmospheric Processes and Air/Sea Interaction
The first lecture on the global view focuses on the atmosphere. This is because the atmosphere to
a large extent drives the ocean. The wind-driven and thermohaline portions of the ocean
circulation are determined by the wind field and the ocean-atmosphere heat exchange processes.
On a smaller scale the atmosphere is a key factor in coastal upwelling, fronts over the shelf and
estuarine exchange. In this chapter we review the global weather system and then focus in on
mesoscale and regional processes We also mention evidence for long term changes in the
atmosphere.
Global Weather Patterns - Conceptual Models
Early researchers realized that there were some fundamental processes in the atmosphere that
controlled the large scale circulation. We present those processes here. The global atmospheric
circulation is basically produced by warm air moving from the equator to the poles and affected
by the Coriolis deflection. Very descriptively, the Coriolis deflection consists of the tendency of
moving objects to be deflected to the right in the Northern Hemisphere (Fig. 1) and to the left in
the Southern Hemisphere. This deflection will be studied quantitatively later.
The earth is heated by the sun most markedly in the tropical regions. The hot air of these regions
is lighter than the cold air of the poles and tends to rise to the upper atmosphere (Fig. 2). Thus,
warm air will move in the upper atmosphere from the tropics toward high latitudes, and cold,
dense air from the poles will tend to move toward the equator near the earth’s surface.
Conceptually, large air circulation cells form in the northern and southern hemispheres.
However, Coriolis deflection will tend to disrupt this large cell as described later.
Sir Gilbert Walker established the concept of a large zonal (east/west) circulation cell in the
Pacific. Walker Cells, as they are called, occur because a great deal of heat absorbed by the
ocean is transferred to the atmosphere in the western Tropical Pacific. This occurs in an area of
strong convection, which is visible in most views of the earth (Fig. 3). The moist air rises and
moves eastward finally descending as a dryer air mass over the eastern Pacific Ocean (Fig. 3).
Note also in Figure 3 the areas of convection over the western Pacific, Congo River basin and
Amazon River basin and the significant area of convergence over the eastern Central Pacific.
The zonal flow of the Walker Cell is modified by the earth’s rotation and meridional flow. This
flow of ascending moist air in the tropics and descending dry air in the subtropics is called the
Hadley Cell. The return flow in the Walker/Hadley Cell System is the trade wind (Fig. 4).
The trades are the persistent strong winds blowing southwestward just north of the Equator and
northwestward just south of the equator. The convergence zone where these winds meet is called
the Intertropical Convergence Zone (ITCZ) and represents the Doldrums. The Doldrums were
dreaded by sailors because of the weak winds and high humidity. A schematic of the Hadley Cell
system and the Ferrel and Polar Cells that have similar characteristics are also shown in Figure
4.
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The ITCZ is particularly important both globally and locally. Note the distribution of the ITCZ
across the global ocean in the image of surface temperature and clouds (Fig. 5) and in the water
vapor image (Fig. 3).
Those are conceptual models that help us understand the real distributions of variables such as
pressure field and wind. They are all summarized in a schematic of winds over the globe (Figure
6). Note how the atmospheric circulation cells drive the surface wind field that is characterized
by the Doldrums, the Trades, and the Westerlies. The global wind field must be understood in
order to interpret the global ocean circulation patterns, such as the major ocean currents, and also
the local ocean circulation patters, such as coastal upwelling.
Because the global wind field and its fundamental variability is determined by the atmospheric
pressure field we present the pressure field first (Fig. 7). Note the low atmospheric pressure
systems associated with the regions of surface air flow convergence (Doldrums and Polar
Fronts). Also note the high atmospheric pressure at the regions of surface air flow divergence
(Horse Latitudes). The global pressure field for January and July (Figure 8) shows the system of
highs and lows circling the globe. Note the permanent highs over the Indian, South Pacific and
South Atlantic Oceans and the permanent low over the Southern Ocean. In contrast the pressure
fields in the northern oceans alternate with a high dominating the North Pacific and Atlantic in
the summer and a low dominating in the winter. The changing strength and position North
Atlantic and Pacific highs and lows determine to a large degree the surface ocean circulation in
those areas. Also note the strong zonal pattern in the tropics and sub-Antarctic.
Vectors of the mean monthly wind field for January (Figure 7) show many of the features we
have just discussed. The zonal tropical easterlies and the mid-latitude westerlies are clear but the
pattern is more complicated and not symmetrical. Note the obvious regions of convergence that
we talked about in the ITCZ.
With the advent of radar techniques from satellite we can now measure wind speed over the
globe. Data from October 1992 are shown in Figure 9. The strength of the trades is especially
noticeable in this figure where averages of nearly 7 m/s exist. The well known strong winds of
the higher northern latitudes being more episodic are as obvious as the powerful winds of the
Antarctic.
Clearly from all these figures you get the impression that winds in the subtropics are strong and
consistent in direction. Thus, when we examine ocean circulation in the tropics and subtropics
we will expect to see strong consistent, well-defined currents. We will also expect to see
seasonally changing currents in the North Atlantic and Pacific because of shifting high and low
pressure regions.
Now we will return to the Intertropical Convergence Zone for some additional discussion since it
will be important in future topics. Recall that the ICTZ is the zone of convergence of the trade
winds near the equator. The ITCZ lies usually north of the equator and can be considered the
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thermal or meteorological equator. Look back at Figures 3 to 7 again and examine the ITCZ.
Hurricanes and Tropical Storms
Tropical storms are formed in the zones of trade winds both north and south of the Equator.
Figure 10 shows the location of formation and Figure 11 shows the paths the low pressure
systems take. Note the asymmetrical distribution of the source regions between northern and
southern hemispheres. This is another example of the effect of the greater land mass in the
northern hemisphere.
Higher Latitude Storm Systems
You have noted the rather coherent structure of the wind fields over the lower latitudes. As you
go to higher latitudes this pattern breaks down and rather than longer scale patterns such as
ITCZ the effect of the constant passage of low pressure regions that travel on the boundary
between the Ferrel and Polar circulation cells dominate. The winds of the lower latitudes have
less variance in speed and direction than the winds of higher latitudes.
Scales of Atmospheric Processes
If there is one thing we want you to remember in this class is that scales of processes are
important. Scales represent the frequency at which energy is varying. If you were tied to the
track of a train just after it passed and it runs every 24 hours you care little of the next few
minutes. You are focusing on the scale of 24 hours. That is the scale that the energy dominates.
The same can be said for spatial scales. If you are hiking across a level plain and find that there
are deep rivers to cross every 50 km you will start to think in terms of 50 km scales and plan your
hiking schedule accordingly.
Summary
Think about the different processes and conceptual models related to global weather systems.
Place yourself mentally in some location and think about what weather processes you should
consider if you had to understand the ocean environment. For example, what would you put on a
list ranked by importance of weather and climate processes to consider if you where assigned a
task of describing the ocean off New Jersey, off San Francisco, off Calcutta, around Hawaii?
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